Stellar black hole

A stellar black hole, also called a stellar-mass black hole, forms when a star collapses under its own gravity. Its mass typically falls between about five and several tens of times the mass of the Sun. While many are the remnants of supernova explosions, other ways of forming them may exist.

According to the no-hair theorem, a black hole is defined by only three properties: mass, electric charge, and angular momentum. The angular momentum of a stellar black hole comes from the conservation of the spin of the star or objects that created it. A massive star inevitably collapses at the end of its life, once its internal energy sources are gone. If the collapsing core is below the Chandrasekhar limit, a white dwarf forms. If it is above that but below the Tolman–Oppenheimer–Volkoff (TOV) limit for neutron-degenerate matter, the result is a neutron star. If the collapsing core exceeds the TOV limit, the collapse continues until a black hole appears. These limits are approximate and can shift depending on the star's chemical makeup and rotation.

The exact maximum mass a neutron star can have before collapsing into a black hole is not fully known. In 1939, the TOV limit was estimated at 0.7 solar masses. By 1996, a different estimate placed the upper mass between 1.5 and 3 solar masses. The heaviest observed neutron star, PSR J0740+6620, discovered in September 2019, has a mass of about 2.14 solar masses.

In general relativity, a black hole could have any mass. The smaller the mass, the denser the matter must be to form one. No known stellar process produces black holes with masses less than a few times the Sun's mass; if such small black holes exist, they are likely primordial. Until 2016, the largest known stellar black hole had a mass of 15.65±1.45 solar masses. In September 2015, gravitational waves revealed a rotating black hole of 62±4 solar masses, formed when two smaller black holes merged. As of June 2020, the binary system 2MASS J05215658+4359220 contains the smallest known black hole, with a mass of 3.3 solar masses and a diameter of just 19.5 kilometers.

Observational evidence also points to two more massive types: intermediate-mass black holes in globular clusters and supermassive black holes at the centers of the Milky Way and other galaxies.

Stellar black holes in close binary systems become visible when matter flows from a companion star toward the black hole. The energy released as the matter falls heats it to hundreds of millions of degrees, producing X-rays. The black hole is thus seen in X-rays, while the companion star is observed with optical telescopes. The energy release is similar for black holes and neutron stars, making them hard to tell apart. Mass estimates come from observations of compact X-ray sources, combining X-ray and optical data. All known neutron stars have masses below 3.0 solar masses, and none of the compact systems above that mass show neutron star properties. This strongly suggests that compact objects above 3.0 solar masses in binaries are black holes. This proof is partly theoretical: no other object is known that could explain these massive compact systems. A direct proof would require observing the orbit of a particle or gas cloud falling into a black hole.

Some binaries are found far above the galactic plane due to black hole natal kicks. The velocity distribution of these kicks resembles that of neutron star kicks. One might expect the momenta to be similar, with black holes moving slower because they are heavier, but that does not appear to be the case. This may be because asymmetrically expelled matter falls back, increasing the black hole's momentum.

Some stellar evolution models predict that black holes cannot form directly from stellar collapse in two mass ranges: a "lower" gap of roughly 2 to 5 solar masses and an "upper" gap of about 50 to 150 solar masses (another estimate for the upper gap is 52 to 133 solar masses). A star's upper mass limit in the current universe is thought to be about 150 solar masses.

A lower mass gap is suspected because few candidates are observed with masses just above the maximum neutron star mass. The existence and theoretical basis for this gap are uncertain. Any black holes found in this range may have come from merging binary neutron stars rather than stellar collapse. The LIGO/Virgo collaboration reported three candidate events in their third observing run with component masses in this lower gap. There has also been a report of a bright, rapidly rotating giant star in a binary with an unseen companion that emits no light, including X-rays.